This invention relates an image processing method of taking in image data by, for example, reading the image on a document with a scanner, performing a specific process on the input image data, and then outputting the image data on paper with an electronic photographic laser printer and an image processing apparatus, such as a digital copying machine, using the image processing method.
With an image processing apparatus, such as a digital copying machine's body dealing with image data, the image data read from a document by a reading device, such as a scanner, is generally digitized, made multi-valued, and processed according to the purpose, and outputted on an output device, such as a laser printer. In this case, because of the characteristics of the scanner acting as the input device and the photosensitive member and laser optical system in the output device, the desired result cannot be obtained without correction.
In general, to correct the characteristic of the entire system, including the characteristics of the scanner of the input device and the photosensitive member and laser optical system in the output device, a gradation correction table used to correct the characteristic of the entire system beforehand is stored in a memory such as a ROM. The characteristic is corrected by referring to the table. In another approach, each image processing device's body is forced to output a thing from which the gradation characteristic can be known, such as a test print. The test print is then supplied to the input device. From the inputted data, a characteristic correction table for correcting the gradation characteristic is formed. The gradation characteristic is corrected to by reference to the table.
Even if the characteristics of the input device, the output device, and the like are obtained beforehand and stored in such a memory as a ROM and a correction is made by reference to the correction data, it is difficult to make a correction appropriate for each image processing apparatus because the characteristics of the scanner, photosensitive member, and laser optical system differ from one image processing apparatus to another.
To make a correction for each image processing apparatus, the gradation data internally generated in the image processing apparatus is printed at the printer section to provide a hard copy. The hard copy is read by the same image processing apparatus that has generated the gradation data. From the read data, the characteristic correction data for correcting the characteristics of the input device and output device is obtained. This approach is very effective theoretically.
Actually, however, when data on each gradation is obtained by generating gradation data, outputting it on the printer section acting as the output device, and supplying the outputted gradation data to the input device, the relationship between the gradation data sent to the output device and that supplied to the input device plotted on the ordinate and abscissa axes is not ideal. Examples of the data are shown in FIGS. 9 and 10.
As shown in the examples, when the data rises sharply or the data contains a part lying as if it were almost parallel with the abscissa, small noise (the portion indicated by "a" in FIG. 9 or the portion indicated by "b" in FIG. 10) may make the characteristic correction data discontinuous or cause the reversal of numerical values in the parts close to the abscissa and ordinate axes.
It is very easy for a person to correct the discontinuity or reversal of the data due to noise, while seeing it with his or her eyes. It is very difficult, however, to do the same thing using calculating expressions. Actually, because of such noises, the characteristic correction data cannot be created exactly.
This causes the problem of being unable to get the linearity as expected theoretically, even if the gradation data is corrected for each digital copying machine.